Travel expenses are a heads-up item for an audit. It's what triggered mine. Here's what you do to audit-proof them.
When I'm on the road I obsessively collect my receipts. I order a Starbucks, I put the cash register receipt in my wallet. When my wallet gets too full, I store them in a zip lock baggie in my luggage.
When I get home I gather my pile of receipts by the computer and start entering each one onto a spreadsheet. I have an Excel template I've created, with simple categories: food, lodging, airfare, transport (cabs, trains), other. I code which expenses I paid with a credit card and which ones I used cash. Then I filter for the cash transactions, and write myself a check for that amount. That's how I capture those expenses into Quicken.
All the receipts, and a printout of the spreadsheet, go into a 9x12 envelope. I label it with the client and the dates of travel, and file it.
Now, what travel can you put on your Schedule C? Most of my travel is for a client. They reimburse me for all my expenses. I have the proof on the invoice. I can deduct every penny.
If no one else is reimbursing your travel, it gets more complicated. You need to take care. Is the travel about your income-producing photography? Or is it a pleasure trip, during which you took some pictures? This is not the place to get creative.
Here's how it went down with the auditor. One of my trips that year was to Maine, to take a week long digital printmaking workshop with John Paul Caponigro. I spent another week on the road photographing and working on my contra dance project. I travelled alone. That travel passed muster.
Later that year I flew down to meet Robin in LA, where she was at a conference. I took photos from the hotel balcony, and those shots are in a stock agency. We drove up the coast together for 5 days, and had a sweet, romantic time. We visited my niece and nephew in San Francisco. I took a couple thousand photos. Some of those are on the Getty site. But I could not demonstrate that the primary reason for the trip was for business, even if some business activity took place during the trip (the rules say exactly that). The auditor wouldn't let me deduct that trip (which was fine—I had already categorized it as a personal trip. If, however, I had taken this trip in 1998, when 90% of my income was from stock photography, I might have been more aggressive about including it. I can't say if I would have been successful.)
Another gotcha with unreimbursed travel is food. You can deduct only 50% of the cost of your meals. Another reason to segregate expenses by type. There are also rules regarding travelling with a spouse. Ask your accountant what they are.
The single most important thing to do? Be honest. Life is so much less stressful that way.